1. Field of the Invention
The present invention relates to a surface acoustic wave device to be used in a resonator, a narrowband filter, a resonant filter, an oscillator, etc.
2. Description of the Related Art
With an increasing demand for small-size portable and cordless telephone sets, a high-performance filter is required to properly process radio signals. The signal is to be small- size and loss and to excel in damping property. A surface acoustic wave device is used in the high-performance filter. A surface acoustic wave (SAW) is excited by applying a electric field to a piezoelectric monocrystal or ceramic substrate, and propagates energy more than 90 percent of which is concentrated to the depth within one wave length from the surface of the substrate.
As compared with a filter using a bulk wave, the filter comprising the surface acoustic wave device can tap a signal from any point in a propagation path, and utilizes the merits of the surface acoustic wave such as controllability of propagation property. When compared with an LC circuit and a cavity resonator, it advantageously has a very high Q value.
FIG. 1 shows a general appearance of a model of a conventional surface acoustic wave device (surface acoustic wave resonator). As shown in FIG. 1, the surface acoustic wave device 1 comprises an inter-digital transducer (IDT) 3 on the surface of an piezoelectric monocrystal substrate 2, and a reflectors 4 mounted on both sides of the inter-digital transducer 3. The reflector 4 is a short-strip reflector. The inter-digital transducer 3 comprises two comb-shaped driving electrodes 3a and 3b mounted as opposed to each other. Electrode fingers 3c of the comb-shaped driving electrode 3a and those of the comb-shaped driving electrode 3b are inserted to one another.
In the above described surface acoustic wave device 1, the relation between the wave length .lambda. of the surface acoustic wave excited by the inter-digital transducer 3 and an inter-electrode distance L3 can be calculated by the following equation. EQU L3=.lambda./2
where normally L3=.lambda./2+n.lambda. (n=0, 1, 2, . . . ) The propagation velocity v.sub.s of the surface acoustic wave can be calculated by the equation v.sub.s =f.sub.0 .multidot..lambda. where f.sub.0 indicates the center frequency of the surface acoustic wave device 1. Thus, the center frequency f.sub.0 can be calculated by the following equation. EQU f.sub.0 =v.sub.s /2.multidot.L3
where v.sub.s indicates the propagation velocity of the surface acoustic wave.
FIG. 2 shows the resonance characteristic of the surface acoustic wave device 1. In FIG. 2, .DELTA.f indicates a passband width. The surface acoustic wave device 1 controls the inter-electrode distance L3 of the inter-digital transducer 3 so that the wave length .lambda. of the surface acoustic wave, that is, the center frequency f.sub.0, can be properly controlled. The passband width .DELTA.f can also be controlled by increasing or decreasing the number of pairs of electrodes (number of pairs of electrode fingers 3c) of the inter-digital transducer 3. The passband width .DELTA.f becomes narrower as the number of pairs of the electrodes increases.
The inter-electrode distance L4 of the reflector 4 is equal to the inter-electrode distance L3 of the inter-digital transducer 3 because a standing wave is generated in the surface acoustic wave device 1 by reflecting the surface acoustic wave of the wave length .lambda.0 excited by the inter-digital transducer 3. The electrode dye width L1 of the comb-shaped driving electrodes 3a and 3b is a half of the inter-electrode distance L3, and the electrode dye width L2 of the reflector 4 is a half of the inter-electrode distance L4. These inter-electrode distances L3 and L4 are hereinafter referred to as "grating cycles" commonly used currently.
In FIG. 1, each of the grating cycles L3 and L4 is shown as the distance between the right-side ends or the left-side ends of the adjacent electrode fingers 3c or 4c. Exactly, they refer to the distance between the centers of the adjacent electrode fingers 3c or 4c. In FIG. 1, the grating cycle is shown such that the relation between the grating cycles L3 and L4 and the electrode finger widths L1 and L2 can be clearly recognized.
The surface acoustic wave device 1 is designed to obtain a resonance characteristic by reflecting the surface acoustic wave excited by the inter-digital transducer 3 on the reflector 4 and by enclosing the surface acoustic wave in the surface acoustic wave device 1. As described above, the passband width .DELTA.f becomes narrower as the number of pairs of electrodes increases. An input signal is input to input terminal a to excite the surface acoustic wave, and input terminal a is connected to the comb-shaped driving electrode 3a. Output terminal b through which an electric signal of a frequency in the passband width .DELTA.f is connected to the other comb-shaped driving electrode 3b. Thus, output from output terminal b are only the components in the passband width .DELTA.f with the center frequency f.sub.0 positioned in the center.
As described above, the surface acoustic wave device 1 encloses energy such that larger amplitude of the surface acoustic wave can be generated in the inter-digital transducer 3 based on the difference in frequency between the surface acoustic waves generated by the inter-digital transducer 3 and the reflector 4. The difference in frequency is generated from the difference in propagation velocity between the surface acoustic waves in the inter-digital transducer 3 and reflector 4. The propagation velocity of the surface acoustic wave differs between the inter-digital transducer 3 and reflector 4 based on the following grounds.
That is, the reflector 4 functions as .DELTA.V/V wave guide path of the surface acoustic waves. The .DELTA.V/V wave guide path comprises strips of conductive films arranged at predetermined intervals on the piezoelectric substrate. The propagation velocity V of the surface acoustic wave is reduced by .DELTA.V through a short circuit on the surface of the wave guide path, thereby obtaining an electric field short-circuiting effect.
The inter-digital transducer 3 is divided into an input unit (comb-shaped driving electrode 3a) and an output unit (comb-shaped driving electrode 3b). The electric short-circuit of the inter-digital transducer 3 is smaller than that of the reflector 4. Therefore, the propagation velocity of the surface acoustic wave differs between the inter-digital transducer 3 and the reflector 4. Thus, the propagation velocity v.sub.d of the surface acoustic wave device in the inter-digital transducer 3 is presumably higher than the propagation velocity v.sub.r of the surface acoustic wave in the reflector 4. The propagation velocity of the free surface acoustic wave on the surface of the piezoelectric monocrystal substrate 2 is higher than that of any portion of the inter-digital transducer 3 and the reflector 4. This also proves the above mentioned electric field short-circuiting effect.
As described above, the conventional surface acoustic wave device 1 shown in FIG. 1 encloses energy such that larger amplitude of the surface acoustic wave can be generated in the inter-digital transducer 3 based on the difference in frequency between the surface acoustic waves generated by the inter-digital transducer 3 and the reflector 4. If there is no difference in frequency between the surface acoustic waves of the inter-digital transducer 3 and the reflector 4, or if no appropriate frequency difference is recognized, then a large amount of energy loss lowers the performance of the device (resonator).
The frequency of the surface acoustic wave on each portion of the inter-digital transducer 3 and the reflector 4 is obtained by dividing the propagation velocity of the surface acoustic wave by its wave length .lambda.. As described above, the wave length .lambda. in the inter-digital transducer 3 and the reflector 4 depends on the grating cycle L3 and the grating cycle L4 respectively. The propagation velocity of the surface acoustic wave in the inter-digital transducer 3 is different from that in the reflector 4. The difference in propagation velocity is made on various conditions such as material of the piezoelectric monocrystal substrate 2, available frequency band, etc. Therefore, it is very difficult to optimally control the surface acoustic waves of the inter-digital transducer 3 and the reflector 4 such that a desirable frequency difference can be made.
There are some methods of adjusting the difference in frequency. For example, the grating cycle L3 of the inter-digital transducer 3 is made different from the grating cycle L4 of the reflector 4 (that is, L3 .noteq.L4 in FIG. 1). Another method is to vary the thickness of the films of the inter-digital transducer 3 and the reflector 4. However, these conventional methods have the following problems.
If the grating cycle L3 of the inter-digital transducer 3 differs from the grating cycle L4 of the reflector 4, then the surface acoustic wave indicates propagation unconformity. Consequently, spurious is increasingly generated, and the energy enclosure efficiency is lowered by suffering from a larger energy loss from increased bulk wave radiation through the promotion of the mode switch from the surface acoustic wave to the bulk wave. The increasing generation of spurious may result in an energy loss and a narrow band width.
The propagation velocity of the surface acoustic wave also varies with the thickness h of the film of the electrode of the inter-digital transducer 3 and the reflector 4. A larger thickness h of the film lowers the propagation velocity (mass load effect). Using this effect, the thickness of the film of the inter-digital transducer 3 can differ from that of the reflector 4. However, this method has had the problem of increasing the manufacturing steps, complicated operations, manufacture time, and entire cost.
The spurious of the surface acoustic wave device 1 can also be generated by converting a part of the surface acoustic wave into a surface skimming bulk wave.
The surface skimming bulk wave has a feature of generating no difference in propagation velocity between a free surface and a short-circuit surface. Since there is a small difference in propagation velocity between the surface acoustic wave and the surface skimming bulk wave, the surface skimming bulk wave indicates a resonant frequency, thereby badly affecting the characteristics of a resonator. Accordingly, in the conventional technology, the thickness h of the film of the inter-digital transducer 3 and the reflector 4 is increased to lower the propagation velocity of the surface acoustic wave. Thus, the difference in propagation velocity is made larger between the surface skimming bulk wave and the surface acoustic wave to reduce the undesirable affection of the conversion into the surface skimming bulk wave. However, increasing the thickness h of the film of the inter-digital transducer 3 and the reflector 4 causes various problems such as low reliability of the film, low stability of film production, and longer operations required to properly form the inter-digital transducer 3 and the reflector 4.
Furthermore, increasing the thickness h of the film also causes uneven thickness h of the film and causes the problem of producing uneven forms of electrodes (for example, an electrode finger) in an etching process. Therefore produced are uneven propagation velocity of the surface acoustic wave, uneven grating cycles, etc. As a result, the stability of frequency represented by, for example, the precision of a center frequency f.sub.0 is undesirably lowered.
There has been the problem to be solved that the conversion from the surface acoustic wave to the surface skimming bulk wave should be reduced because the surface acoustic wave device 1 has a lower energy enclosure efficiency when the surface acoustic wave is converted to a large extent into the surface skimming bulk wave with an increasing generation of spurious.